Lithostratigraphy of the Palaeogene – Lower Neogene succession of the Danish North Sea

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Geological setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Previous work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Material and methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Offshore and onshore lithostratigraphic nomenclature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Chronostratigraphy and biostratigraphy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Paleocene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Eocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Oligocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Miocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Lithostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Rogaland Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Våle Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Bor Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Lista Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Vile Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Tyr Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Ve Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Idun Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Bue Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Rind Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Sele Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Kolga Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Fur Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Balder Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Stronsay Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Horda Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Hefring Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Westray Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Lark Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Dufa Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Freja Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73


Introduction
Intense drilling activity following the discovery of the Siri Field in 1995 has resulted in an improved understanding of the siliciclastic Palaeogene sediment package in the Danish sector of the North Sea (Fig. 1). Many of the new wells were drilled in the search for oil reservoirs in sandstone bodies of Paleocene-Eocene age. The existing lithostratigraphy was established on the basis of data from a generation of wells that were drilled with deeper stratigraphic targets, with little or no interest in the overlying Palaeogene sedimentary succession. This means that this early scheme does not include Palaeogene sandstone units in the Danish sector. In order to improve the understanding of the distribution, morphology and age of the Palaeogene sediments, in particular the economically important sandstone bodies, a detailed study of this succession in the Danish sector has been carried out. The main aim was to update the lithostratigraphic framework of the succession on the basis of new data from recently drilled wells. All of the widespread Palaeogene mudstone units in the North Sea were established with Norwegian or United Kingdom (UK) type wells. In the present work, these units have been maintained unchanged or with only slight modifications. Danish reference wells have been established for the units, however, and lithological descriptions have been expanded to cover the characteristics of these units in the Danish sector.
Many of the sandstone bodies recently discovered in the Danish sector have a limited spatial distribution and are derived from sources different from those of most of the contemporaneous sandstone bodies in the Norwegian and UK sectors; furthermore, the Danish sandstone bodies probably neither overlap nor are in contact with the Norwegian/UK sandstones. These units have therefore been established as new in the Danish sector, and have been assigned Danish type and reference sections.
The lithostratigraphy presented herein (Fig. 2) has its base at the top of the Early Paleocene (Danian) Ekofisk Formation (Chalk Group). The top of the study section is at the unconformity between the Late Eocene -Mid-Miocene Westray Group and the Mid-Miocene to Recent Nordland Group.
Oil companies operating in the North Sea have collected a substantial amount of lithostratigraphic data on the Palaeogene successions and a detailed lithostratigraphy has been developed for the Danish and Norwegian sectors (see e.g. Hamberg et al. 2005). A number of informal lithostratigraphic units have been introduced that have subsequently found their way into academia and geological survey organisations. It has been the aim of the present work formally to define these new units. This has been done maintaining their original (albeit informal) names whenever feasible.
It has not been the aim of this work to provide a sequence stratigraphic model for the Palaeogene sediments in the central and eastern North Sea; for this the reader is referred to Michelsen et al. (1992, 1995, 1998), Mudge & Bujak (1994, 1996a, b), Neal et al. (1994 and Danielsen et al. (1997). The present contribution does not attempt to review the petroleum-related aspects of the Palaeogene succession. Information about this may be found elsewhere, for example in the annual reports from the Danish Energy Authority.
Preliminary results from the present work, including a revised lithostratigraphic scheme, were previously published in a brief review paper (Schiøler et al. 2005). The present contribution formally describes the new stratigraphic units suggested in the review paper and further documents the Palaeogene -Lower Neogene lithostratigraphy in the Danish sector of the North Sea.

Geological setting
The Danish sector of the North Sea is situated in the central and eastern North Sea and comprises three major structural elements: the Central Graben, the Norwegian-Danish Basin (the eastern part of the northern North Sea Basin of Rhys 1974) and the Ringkøbing-Fyn High ( Fig. 1; the geographic terminology and names of structural elements in the North Sea used herein are adapted from Rhys 1974, Rønnevik et al. 1975, Deegan & Scull 1977and Fyfe et al. 2003. The western boundary of the Danish sector largely coincides with the eastern boundary of the Mid North Sea High, the southern boundary largely coincides with the southern limit of the Ringkøbing-Fyn High, and the northern boundary is in the Norwegian-Danish Basin. This basin as well as the Ringkøbing-Fyn High are Early Permian structures. Active rifting occurred in the Central Graben from the Middle to Late Jurassic along pre-established Palaeozoic fault trends. Major tectonic activity around the Palaeozoic and Jurassic structures had largely ceased by Late Cretaceous time, and the sediment basin below the central North Sea was largely characterised by regional subsidence (Ziegler 1981). During the Late Cretaceous to Danian sea-level high, pelagic chalk sediments draped the structural highs and the northern and southern North Sea Basins became one North Sea Basin delimited by the Fennoscandian Shield to the north-east, the Rheinish-Bohemian Massif to the south and the British massifs, highs and platforms to the west (see Ziegler 1981 fig. 16 for details). Chalk sedimentation continued through to the end of the Danian Stage when it gave way to hemipelagic and siliciclastic sedimentation. This was probably caused by uplift of the basin margins to the west and east (Ahmadi et al. 2003). However, most of the siliciclastic sediments were derived from the Scottish High and the East Shetland Platform, uplifted by the Iceland plume (Ahmadi et al. 2003). By the time of peak uplift, in the mid-Thanetian, large sand systems were building out towards the central North Sea. Most sediment came from the west, but the Siri Canyon system, a depression in the top chalk surface, was fed from the Fennoscandian Shield in the north-east and north (Fig 1;Ahmadi et al. 2003;Hamberg et al. 2005).
Thermal subsidence centered above the Central Graben continued through the Eocene as sea level fell and the temperature decreased. Shallow-marine sediments characterised the margins of the North Sea Basin, especially its western margin, whereas basinal mudstone continued to accumulate in the basin centre and in the eastern part of the basin (Joy 1996). Inversions controlled by compression between the Atlantic spreading zone to the northwest and the orogenesis of the Alps to the south added to further uplift of the basin margins and submarine fans and turbidites were deposited near the centre of the basin (Jones et al. 2003).
During the Oligocene, the North Sea Basin became part of a larger NW European basin. Connection with the North Atlantic broadened and enhanced communication with the oceanic water mass to the north-west, whereas the connection to the south through the North Polish Strait became closed for the deep water (Fyfe et al. 2003). Glacio-eustatic sea-level changes became more frequent and controlled the sedimentary cycles. The eastward progradation direction of the Paleocene and Eocene sediments gave way to sediment supply from the European massifs to the far south (Fyfe et al. 2003). Continued subsidence above the Mesozoic rift structures created accommodation space for thick sediment packages of basinal mudstones, and few sandstone units reached the basin depocentre above the Mesozoic rifts (Fyfe et al. 2003). In the Neogene Epoch, sediment started to be derived from the Fennoscandian Shield to the north, and the progradation direction changed to the south-west and west in the Danish sector of the North Sea.

Previous work
The Permian to Recent lithostratigraphy of the North Sea was described in two pioneering stratigraphic works. Rhys (1974) provided an overview of the structural elements of the North Sea and gave a brief description of the Palaeogene sediments. Deegan & Scull (1977) compiled a detailed lithostratigraphic subdivision and lithological description for the central and northern North Sea (Figs 3,  4). They subdivided the siliciclastic Palaeogene, Neogene and Quaternary sediments into five major groups: the Montrose, Moray, Rogaland, Hordaland and Nordland Groups. The Montrose and Moray Groups established for the Outer Moray Firth -Forties area are proximal equivalents to the Rogaland Group and are not present in the Danish sector, whereas the Rogaland, Hordaland and Nordland Groups have widespread distribution in the Danish sector. The succession of major mudstone formations contained within the three basinwide groups has formed the backbone of all subsequent lithostratigraphic schemes for the central and northern North Sea, including that of the present contribution.
The post-Danian Cainozoic succession of the Danish Central Graben was divided into seven informal units by Kristoffersen & Bang (1982). The Palaeogene comprised five units: North Sea Marl and CEN-1-4 (Fig. 4). The ranks of the units were not stated. Although descriptions and interpretation of the CEN units were detailed, they are essentially informal and have been little used.
A revised lithostratigraphy for the Palaeogene and Neogene of the Norwegian North Sea sector was published by Hardt et al. (1989). Their lithostratigraphic scheme includes a number of new Palaeogene and Neogene sandstone bodies observed in the Norwegian and British sectors of the North Sea (Fig. 4). Some of the names of the new sandstone units established by Hardt et al. (1989) were subsequently used informally for comparable sandstone units discovered in the Danish sector.
Mudge & Copestake (1992a, b) presented a revised Palaeogene stratigraphy for the Outer Moray Firth and northern North Sea Basins. In their papers they redefined the Moray and Montrose Groups of Deegan & Scull (1977) and abandoned the Rogaland Group. The authors also demoted the previously established sandstone formations within the two former groups to the rank of members. Besides, in an innovative approach they allowed for a greater influence of biostratigraphic data on the characterisation of the various lithostratigraphic units, an approach which is also followed herein.
Knox & Holloway (1992) updated the lithostratigraphic scheme for the Palaeogene in the British and Norwegian central and northern North Sea (Figs 3, 4). The authors followed Mudge & Copestake (1992a, b) in abandoning the Rogaland Group of Deegan & Scull (1977), and used Mudge & Copestake's revised definition of the Montrose and Moray Groups for the central North Sea as well. Furthermore, the thick and hitherto undivided Hordaland Group was subdivided into two new groups, the Stronsay Group succeeded by the Westray Group, each containing a distal and a proximal formation. The two distal formations of the two groups, the Horda and Lark Formations, together constitute the bulk of the Palaeogene sediments in the Danish sector of the North Sea and are adopted herein (Figs 2-4). Although sandstone units occur in both the Horda and Lark Formations in the Danish sector, the two proximal sandstone formations of the  Stronsay and Westray Groups, the Mousa and Skade Formations, are absent from the Danish sector. Following detailed analysis of new, high-resolution seismic surveys covering the succession in the eastern North Sea area, efforts were focused on establishing a sequence stratigraphic subdivision of the Palaeogene-Neogene sediment package. The sedimentary succession was interpreted in a series of publications from a working group at the University of Aarhus (e.g. Michelsen et al. 1992Michelsen et al. , 1995Michelsen et al. , 1998Michelsen 1993;Danielsen et al. 1997;Huuse & Clausen 2001). The result of that work was a subdivision of the Palaeogene to mid-Neogene sediment package covered by the present work into six genetic units (Fig. 4). The sequence stratigraphy of the upper Oligocene to Miocene in the eastern North Sea was dealt with by Rasmussen (2004b). Further sequence stratigraphic contributions covering the larger North Sea Basin including the British and Norwegian sectors are given by Armentrout et al. (1993), Mudge & Bujak (1994, 1996a, b) and Neal et al. (1994.

Material and methods
The present lithostratigraphic subdivision represents the combined results from studies of petrophysical logs, biostratigraphy and seismic profiles, cuttings samples and cored sections. Petrophysical logs from c. 70 wells in the Danish sector have been scrutinised (see Fig. 1 for well locations). The wells have been correlated using petrophysical logs, predominantly gamma-ray and sonic logs. Five log panels form the basis for the log correlation (Plates 1-5). Lithostratigraphic well correlation has been supported by biostratigraphic data: biostratigraphic reports from 29 wells have been re-assessed with the aim of identifying key micropalaeontological and palynological events that occur consistently within the study area (taxa used are planktonic and benthic foraminifers, diatoms, radiolaria, sporomorphs and dinoflagellate cysts). Moreover, biostratigraphic sample suites from 11 North Sea wells have been prepared at the Geological Survey of Denmark and Greenland in order to further determine the biostratigraphic event succession. The bulk of material studied for biostratigraphy is based on cuttings samples, and only few    et al. (2003) for the Sparnacian-Ypresian boundary. Key dinoflagellate datums are calibrated mainly using age estimates from Hardenbol et al. (1998) andWilliams et al. (2004). Key microfossil datums are calibrated via their correlation with calibrated dinoflagellate datums as suggested by Mudge & Bujak (1996b), using age estimates from Hardenbol et al. (1998) andWilliams et al. (2004). The combined event succession is correlated with the North Sea microfossil zonation of King (1989) and lithostratigraphic units treated herein. In the microfossil event column, the planktonic foraminifer events appear in normal font, benthic foraminifers in italics; diatoms and radiolarians are underlined. core samples have been available. As the use of stratigraphic lowest occurrences (LO) of taxa in cuttings samples may be hampered due to downhole caving, the event succession comprises almost exclusively stratigraphic highest occurrences (HO) of taxa (a single significant LO is included in the succession). The event succession is shown in Fig. 5a-c; its correlation with international and North Sea biozones is shown in Fig. 6a-c. Seismic sections from the 2-D and 3-D seismic surveys CGD85, DK-1, RTD81-RE94, UCG96 and UCGE97 have been used to further support the well correlation and to map the stratigraphic units in areas with only scattered well coverage. The combined results from the correlation and mapping procedures are presented as isochore maps for individual stratigraphic units.
Inspection of cuttings samples from 16 key wells supplemented with sedimentological studies of cored inter-vals from 23 wells have formed the basis for the lithological and sedimentological descriptions of the units.
The well depths mentioned in the lithostratigraphy section are loggers' depths measured either from rotary table (MDRT) or kelly bushing (MDKB). Supplementary data for new type and reference wells are provided in Table 1.
The names assigned to the new lithostratigraphic units defined herein are derived from Nordic mythology and thus follow the nomenclatural tradition previously established for the Norwegian North Sea (Isaksen & Tonstad 1989).
It should be noted that the micropalaeontology-based palaeoenvironmental terminology used herein was originally developed for a passive margin situation (e.g. the terms 'neritic' and 'bathyal' to indicate the physiographic zones 'shelf' and 'shelf-slope', respectively). Its application herein to the epicontinental North Sea Basin solely relates to depositional depth.
Offshore and onshore lithostratigraphic nomenclature